This invention relates to a motor control system, a method of operating a motor control system, a tape drive including a motor control system, a method of operating such a tape drive and a printing apparatus including such a tape drive.
Such printing apparatus includes drive apparatus for moving the tape relative to the printhead, to present fresh tape, from which pixels of ink are yet to be removed, to the printhead, such that successive printing operations can be carried out. It has long been known to provide tape drives which include two spool supports, one of which supports a supply spool on which unused tape is initially wound, and the other of which supports a take-up spool, onto which the tape is wound after it has been used. Tape extends between the spools in a tape path. Each of the spool supports, and hence each of the spools of tape, is drivable by a respective motor.
It is known to provide thermal transfer printing apparatus in two different configurations. In the first, so called “intermittent” configuration, the substrate to be printed and the tape are held stationary during a printing operation, whilst the printhead is moved across the area of the substrate to be printed. Once the printing operation is complete, the printhead is lifted away from the tape, and the tape is advanced to present a fresh region of tape to the printhead for the next printing operation.
In the second, so called “continuous” configuration, the substrate to be printed moves substantially continuously and the tape is accelerated to match the speed of the tape before the printhead is brought into thermal contact with the tape and the printing operation is carried out. In this configuration, the printhead is maintained generally stationary during each printing operation.
It is known to interlace images, such that a previously used region of tape is reused, but in the second and/or subsequent printing operations, different pixels of ink are removed from the tape to create an image. In the case of a printing apparatus in continuous configuration, it is also preferable to accurately control the speed of the tape, to ensure that it matches the speed of the substrate. A typical thermal transfer printer operates with a substrate that advances at linear speeds between approximately 0.01 meter per second and approximately 2 meters per second. Typical substrate accelerations are up to approximately 12 meters per second per second.
Tape drives of various types have been proposed, for example a tape drive which includes a stepper motor for driving a take up spool so as to pull tape through along a tape path between a supply spool and the take up spool. Such a tape drive also includes a mechanical clutch for setting and maintaining the tension in the tape. Such tape drives are often mechanically complex and regular maintenance of the clutch is typically required. Furthermore, since the supply spool is operated at a fixed torque, the tension in the tape varies as the diameter of the supply spool varies over time.
Another example of a known tape drive is one in which a take up spool and a supply spool are rotated by respective stepper motors. The stepper motors are driven in a co-ordinated manner to transfer the tape from the supply spool to the take up spool and to accurately position the tape adjacent the printhead, whilst maintaining the tension in the tape. Various methods of determining and maintaining the tension of the tape have been proposed. Such methods typically require the measured tension in the tape to be compared with the desired tension, and for a correction to be applied. Therefore, such methods often incur a delay of at least one printing operation between the tension in the tape falling outside an acceptable range and the correction being applied.
A further example of a known tape drive includes a pressure roller in the tape path, which is driven by a motor. The roller directly controls the speed and position of the tape. The tape spools are driven through a mechanical clutch which maintains the tape tension between acceptable limits. Such tape drives are often mechanically complex. The tape drive is typically uni-directional and this tends to cause tape wastage.
A still further example of a known tape drive is one in which two DC motors are used to drive the spools of tape (as described in FR 2783459, for example). Both of the motors operate in torque control mode, and a roller which is positioned near to the printhead is used to determine the movement of the tape along the tape path. Such a tape drive includes rollers on the inked side of the tape which can require regular maintenance. Furthermore, desired printing speeds and tape accelerations are increasing, leading to difficulties in operating such a drive.